A 'caged' biotin molecule is used to pattern proteins onto a waveguide surface. Upon irradiation with the appropriate ultraviolet light, the caging group is removed and the biotin becomes active.It can then be used to bind streptavidin and subsequently biotinylated antibodies for immunoassay. SIMS is being employed to demonstrate that the caging group has been removed, and to image the patterned surface by identifying regions of bound streptavidin.